ethtool: Update ethtool_rxnfc::rule_cnt on return from ETHTOOL_GRXCLSRLALL

A user-space process must use ETHTOOL_GRXCLSRLCNT to find the number
of classification rules, then allocate a buffer of the right size,
then use ETHTOOL_GRXCLSRLALL to fill the buffer.  If some other
process inserts or deletes a rule between those two operations,
the user buffer might turn out to be the wrong size.

If it's too small, the return value will be -EMSGSIZE.  But if it's
too large, there is no indication of this.  Fix this by updating
the rule_cnt field on return.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/intel/ixgbe/ixgbe_ethtool.c

@@ -2283,6 +2283,8 @@ static int ixgbe_get_ethtool_fdir_all(struct ixgbe_adapter *adapter,
 		cnt++;
 	}
 
+	cmd->rule_cnt = cnt;
+
 	return 0;
 }